Human and animal vaccines have been commercially produced by growing the desired virus in primary cells. The initial commercial processes involved the use of large quantities of individual bottles having cells growing on the inner surface of the bottles. As production techniques evolved, the original bottles were replaced by roller bottles which somewhat reduced the number of bottles required and the handling problems associated therewith.
More recent mass cell culture systems involve the multi-plate unit disclosed in U.S. Pat. No. 3,407,120 and the Biotec cylindrical rotating disc apparatus. In all mass cell culture systems, however, it is a requirement that the cells be aerated. It has become commonplace to aerate the system with a mixture of air and CO.sub.2 thereby aerating the system and controlling pH at the same time. Typically, a mixture of 95% air and 5% CO.sub.2 is used. Due to the large area involved in mass cell culture systems, it is nevertheless difficult to achieve the desired pH control.